Building a car from the ground up is very difficult, and requires many people with many types of skills. Additionally, engines are almost prohibitively expensive to design, so you may want to use a crate engine to power your custom vehicle.
Constructing the Body
The body is the easiest part of the vehicle to build. It can be done as an entire body shell like the Wikispeed C3, or in many sections, so if one of the sections is damaged, you don't have to replace the entire body.
Building the Chassis
If you intend to sell a car like this, it is best to start with a chassis from another car, because most countries have laws which would require testing, which would mean you would have to build multiple copies of the car you wish to sell.
Making the Engine
Building an engine is possible, but very difficult without the proper machinery, engineering, and metal.